Bill Clinton, who remains one of the most popular Democrats in western Pennsylvania, threw his support behind Rep. Mark Critz over fellow Rep. Jason Altmire in the primary for a House district north of Pittsburgh.

“I am proud to endorse Mark Critz for Congress,” Clinton said in a statement provided by the Critz campaign. “I know that Mark will continue his work to create jobs, strengthen the middle class, to protect Social Security and Medicare, and do what is right for western Pennsylvania and our nation.”

Critz's campaign is using the endorsement in a new advertisement set to air this evening, less than two weeks before the April 24 primary. Altmire has a geographical advantage in the race, but Clinton's backing aims to boost Critz in the final days.

So why would a former president wade into a Member-vs.-Member Democratic Congressional primary? Clinton and his wife have some history with Altmire, according to a Democratic source and Critz supporter with knowledge of the events.

When Hillary Clinton ran for president in 2008, Altmire aggressively pursued time with the political power couple during their repeated trips to the region. He hemmed and hawed over whether to endorse before his home state primary, and he eventually requested to ride along with the former president on the 45-minute drive between events in Hermitage and Beaver Falls, Pa.

"If someone requests that, then you think they’re close to giving you their backing," the source said. "And [Altmire] never endorsed anybody.”

Fast-forward four years. On Friday, Critz's camp got word he would receive Clinton's backing. It's also worth noting that Critz's predecessor and former boss, the late Rep. John Murtha (D-Pa.), was a Clinton supporter in the 2008 race.

“It is an honor to be endorsed by President Bill Clinton because he knows what it takes to turn an economy around and put people back to work,” Critz said in a statement.

The winner of the Democratic primary will face the likely GOP nominee, attorney Keith Rothfus, this fall.